This invention relates to swept Frequency Oscillators. In general, the output power of an oscillator changes as its frequency is varied over its operating band, but it is usually required to produce a substantially constant output power level as the frequency changes. This can be achieved by monitoring the actual output power generated and controlling it by means of a feedback loop so that the level is substantially constant. In order to provide satisfactory operation, the time constant of the power monitor must be large in comparison to the period of the frequency being monitored. This requirement can seriously restrict the sweep rate of a swept frequency oscillator and the difficulty is particularly acute for oscillators which sweep over a very wide frequency range, since the sweep rate is determined by the lowest frequency output of the oscillator. This invention seeks to provide an improved swept frequency oscillator in which this disadvantage is reduced.
According to this invention, a swept frequency oscillator includes a variable frequency oscillator whose output frequency is capable of being swept over a particular frequency range; a signal level detector arranged to monitor the output signal of the variable frequency oscillator; and means operative over at least part of the frequency range of said oscillator for controlling the time constant of the signal level detector in dependence on the actual frequency value, so that the time constant is at least approximately inversely related to the frequency value.
The invention is particularly applicable to swept frequency oscillators, which sweep over a very wide frequency range. For example, spectrum analysers generally include a swept frequency oscillator and it may be required to sweep from typically less than 100 Hz up to 1 GHz. The power level of the output signal generated by the swept oscillator is chosen primarily in relation to the power handling capability of the circuit or instrument to which the variable frequency is being fed, and to enable accurate measurements to be performed the output level of the swept frequency oscillator must be held at a predetermined constant value irrespective of the frequency. It is common to detect the power level by means of a suitable level detector, such as a diode, and to compare a d.c. control signal generated at the diode with a reference voltage. Any resulting difference signal is indicative of an error in power level and is amplified and applied to a gain controlling element associated with the source of the oscillation, so as to level the output signal by normal feedback action. It is necessary for an integrating time constant to be associated with the level detector, so that the feedback loop does not follow the cyclic amplitude variations of the frequency currently being generated. In practice, this means that the time constant of the level detector must be many times the period of the frequency being monitored. It will be appreciated that at the low end of the frequency band when a signal of about 100 Hz is being generated that the period of the signal is 10 milliseconds and consequently, the time constant associated with the level detector may be as long as 1000 milliseconds. A consequence of this is that the rate at which the oscillator output changes frequency must be relatively slow, since otherwise the frequency would change at a rate which was too rapid for the level detector to respond to in a satisfactory manner. This difficulty is reduced in accordance with the present invention.